Stiffness & Compliance

The bottom line: Unless a design uses an effective suspension system, side-to-side stiffness and vertical compliance/comfort will be directly linked. In almost a 1:1 ratio and regardless of material, as a frame gets stiffer side-to-side, it becomes stiffer vertically as well and thus transmit more road shock. Stiffness of your frame and components relates to how efficiently the energy and power from your body gets to the road and powers you forward. While a small piece of total energy use, stiffer frames and components transmit power more efficiently. That being said, unless you are just doing uphill time trials, don’t compromise comfort or your aerodynamics for small gains in stiffness.

How to go about it: Get properly fit and comfortable first. Then, concentrate on matching equipment up with your position, weight, riding style, power output and individual needs. Larger and/or more powerful riders need stiffer components and frames than smaller and/or less powerful riders. Be aware that a production frame in a large size will be softer than the same model in a smaller size, thus what might be overly stiff for a small rider, might be too soft for a bigger rider. Make sure a design addresses your individual needs.

*Comprehensive studies have not been completed to show exact importance of all variables in relation to each other. Results are estimates from a variety of research studies within the cycling industry.

Explanation and Tech talk:

Much of the bicycle industry has done a good job of creating the impression that different materials offer different ride characteristics. Aluminum is supposed to be stiff and light, but is also known for diminished durability and harsh ride quality; Titanium is supposed to be light, durable, comfortable and compliant, but a little flexible; Carbon fiber is supposed to be light and comfortable while simultaneously enhancing drive train stiffness; Steel (Chromoly) is supposed to be “real” and provide a comfortable and snappy ride, but is known to be a bit heavier and more flexible than other options. Right?

Not necessarily.

All manufacturers are trying to build that perfect combination of ride characteristics where stiffness and responsiveness are maximized, while the ride is still kept silky smooth and comfortable. It is not too hard to find claims of a frame being stiff, yet compliant and comfortable, with fantastic vibration damping characteristics. However, the bicycle industry has never had a good baseline testing protocol to quantify how various materials and designs actually perform in regards to specifics like stiffness and comfort. Everything has pretty much been based on “feel”, which is not a very scientific or reliable way to test a piece of machinery. Automobiles provide a good model for how unreliable “feel” can be. A BMW 745i can cruise along the Interstate at 95 mph without feeling like it is going that fast, while a compact Ford Aspire will comparatively feel like it is going pretty fast at 95 mph. Likewise, a bicycle frame that is really stiff and transmits a lot of road shock, can feel fast while a frame that feels more comfortable and compliant can feel slower. However, as the car analogy demonstrates, such feelings can be misleading. I was involved in a test that was designed to find out a little more about what the reality behind the materials and designs is. We tested the stiffness of some common frame designs and material applications in both horizontal (power transfer) and vertical (comfort and compliance) plane. Some of the test results are below:

Torsional Stiffness of the Rear Triangle: This test applied pressure to the frame’s rear triangle side-to-side and measured how far the frame deflected in inches (moved) under a set pressure. The lower the number, the stiffer the bike is side to side, the less flex it will have, and the more direct the rider’s power will be transmitted to the drive train.

Vertical Frame Compliance: This test was conducted in a similar fashion to the torsional stiffness test, but it measured vertical deflection in inches. The numbers directly relate to a frame’s comfort and ability to absorb vibration. In this case, the higher the number, the more flexible, compliant and comfortable a frame’s rear triangle will be up and down.

The results of the tests demonstrated a correlation between vertical compliance and torsional stiffness. With little variance, and the notable exception of the one suspension frame we tested (Softride Rocket R1), the frames that were stiffer torsionally were also stiffer vertically and the frames that were more compliant vertically were softer torsionally. There was also a good deal of range within materials depending upon their application in design. For example, both the Kestrel KM40 and the Trek OCLV 110 are made of carbon fiber, however the seat tubeless KM40 was softer in both the vertical and horizontal plane than the seat tube equipped Trek OCLV 110. Likewise, the Titanium Serotta Legend Ti OS, which was specifically engineered for bigger riders, was one of the stiffer frames in the test while the Titanium Litespeed Tuscany was one of the most flexible.

The Rinard Test was the most comprehensive tests in this regard. Visit this site for more information on theRinard Test and a detailed breakdown on the frames tested. What the Rinard test found corresponded with our testing. They found that the material itself matters little in regards to torsional stiffness and vertical compliance (responsiveness and comfort). What does matter is the size, shape and wall thickness of the tubing used and the manufacturing technique (carbon lay-up, lugged or welded…) and design of the frame.

There are no bad frame materials – there are only poor applications. Any material can be built to have characteristics that are on the other end of the spectrum of what is commonly thought. Aluminum can be soft and flexible (you may remember aluminum frames made by Vitus in the ‘80’s and early ‘90’s) and Titanium and carbon can be made so stiff and harsh that they would be unrideable. So, why do materials each have their own reputations in regards to ride characteristics? Certain materials lend themselves to certain production designs and it is these initial designs that deserve the credit, or the rap, for a material’s general ride reputation, not the material itself.

When choosing a frame or new bike, do not spend time making judgments about ride quality based upon the materials used to build a frame. Instead, approach your frame decision as an individual. Only consider frame options that fit you well, and then look at the design details and tubing to find the ride characteristics that will best match your needs, body and riding style. Finally, don’t forget that a bicycle is a sum of its parts. The other components (especially the wheels and the fork) that you use effect the way it will ride as much as the frame does and should be chosen based upon how they relate to the other parts around them. If you remove yourself from the advertising claims and choose your bike through a process that considers the big picture, I can promise that you will be happy with the long-term results of your new ride.

Important Considerations for Bigger or Smaller than Average Riders (under 150lbs and over 170lbs):

Keep in mind that most production frame tubing is designed for the “average” rider – usually a male who fits on a 55cm frame and weighs around 160 lbs. As production frames become bigger or smaller than this, or a rider heavier or lighter, the ride quality of the frame is going to change too. For better or for worse, when compared to the spec size (usually about a 55cm) a smaller than average production frame is going to be stiffer and less compliant while a larger than average production frame is going to be softer and more compliant.

If you are a larger than average rider, you need to be cautious of many of the more vertically compliant (more flexible) rigid frame designs on the market. While frames like Kestrel’s KM40 or a Litespeed Tuscany might be a good option for a lighter rider, as a larger more powerful rider, you could over-flex it. This can not only prematurely fatigue the frame but can also lead to shifting and stability issues while sacrificing your power because of too much flex. Lighter riders want to be wary of stiffer frame options as they become even more stiff in smaller frame sizes and a lighter rider simply does not have the mass to flex a stiff frame the way a heavier rider does. A Cannondale with its oversized tubing might not be the best decision. Without flex, a frame will transmit a lot of road vibration and will not be very comfortable. This is one reason we often recommend custom builders like Serotta, who not only build custom geometry frames, but also custom tune the ride by offering a variety of tubing size and shape to match your specific needs and frame size. It is also why, when a production frame will fit the rider well, we often recommend Softride. Beam suspension is the only design that currently eliminates material dependency and allows for a frame that is very compliant and simultaneously quite stiff side-to-side.

When looking at designs, keep in mind that the ride quality a frame is known for is usually based upon the experience of an “average sized” rider. What can ride great under a 160 lbs rider, might be too mushy for a heavier rider or might be too stiff and uncomfortable for a smaller rider. If you are bigger or smaller than average cyclist, it is even more important to approach frame and component decisions based upon your individual needs so that you don’t end up with a bicycle that is too stiff or too soft for your size and power.

From first time riders to Olympians, Ian has helped thousands of athletes achieve their cycling and triathlon goals. Ian develops much of the Fit Werx fitting and analysis protocols and is responsible for technology training and development. He is regarded as one of the industry leaders in bicycle fitting, cycling biomechanics and bicycle geometry and design. He is dedicated to making sure the Fit Werx differences are delivered daily and provides Fit Werx with corporate direction and is responsible for uniting our staff and initiatives.